WGS 84 Explained: The Datum Every GPS Uses

WGS 84 is a geodetic datum defined by an ellipsoid (semi-major axis 6,378,137 m, flattening 1/298.257223563), an origin at Earth's centre of mass, an axes orientation tied to the IERS Reference Pole, and the EGM2008 gravity model. It is the reference frame broadcast by every GPS satellite.

Every GPS satellite broadcasts its ephemeris in WGS 84. Every GeoJSON file is in WGS 84. Every modern web map expects WGS-84 input. The reference frame anchoring all of this is the subject of this article: what WGS 84 actually defines, the four defining parameters and the derived quantities, the seven realizations since the original 1984 frame, the relationship with ITRF and NAD83, and the heights question that the gravity-field model EGM2008 settles. The companion pillar /learn/what-is-a-geodetic-datum covers the datum concept more broadly; /learn/coordinate-systems-overview covers the CRS hierarchy.

What WGS 84 defines

Per ISO 19111:2019, a datum combines a reference ellipsoid, an origin, an orientation, and an associated gravity-field model. WGS 84 specifies all four explicitly.

The four-part definition is what makes WGS 84 a datum rather than an ellipsoid. The ellipsoid alone is a mathematical shape; the datum fixes the shape against the rotating Earth and provides the gravity model needed to relate ellipsoidal heights (above the smooth ellipsoid) to orthometric heights (above the geoid, the gravity-equipotential surface that approximates mean sea level).

The four defining parameters

Everything else — the semi-minor axis, the eccentricities, the normal-gravity formula coefficients, the mean radii — is derived from these four numbers. The full derived set lives on the /reference/wgs84-parameters reference page.

The polar radius is 21.385 km less than the equatorial radius — a direct consequence of the centrifugal flattening of a rotating self-gravitating fluid (Newton predicted this in Principia, 1687). The 21.385 km gap is also the reason 1° of latitude is not constant: the local radius of curvature is largest at the poles, where the surface is flattest in cross-section.

Realizations and epochs

WGS-84's definition has been stable since 1984. Its realization — the actual coordinates of a worldwide network of reference stations against which everything else is measured — has been re-derived seven times to maintain alignment with the IERS International Terrestrial Reference Frame (ITRF).

The G-week labels are GPS week numbers at the introduction of each realization (a useful timestamping convention because GPS time is the operational clock of every modern receiver). Each new realization is tied to a specific epoch because the underlying ITRF is itself epoch-tagged: continental plates drift 2-9 cm per year, and an accurate reference frame must record when a measurement was made.

For sub-metre work that cares which realization a coordinate is in, the modern practice is to record it explicitly — "WGS-84 (G2139) epoch 2021.0" rather than just "WGS-84." Consumer GPS work routinely conflates the realizations without measurable consequence because the differences (~1 cm) are smaller than civilian-GPS positioning error (~4.9 m, per GPS.gov SPS PS 2020). Surveying, geodesy, infrastructure monumentation, and tectonic-velocity work cannot.

WGS-84 vs ITRF

ITRF is the civilian/scientific reference frame, maintained by the IERS through continuous observation of the global GNSS, Very Long Baseline Interferometry (VLBI), satellite laser ranging (SLR), and DORIS networks. WGS-84 is the US Department of Defense's reference frame, maintained by NGA.

The practical relationship: ITRF is the "ground truth" that WGS-84 is calibrated against. NGA observes WGS-84 stations, the IERS publishes the contemporary ITRF, and NGA re-realises WGS-84 to match the ITRF on a roughly decadal cadence. The next realization will likely align WGS-84 with ITRF2020 (released 2022).

WGS-84 vs NAD83

NAD83 is the official datum of US federal mapping. NAD83 is fixed to the North American tectonic plate, while WGS-84 is fixed to the global Earth-centre-of-mass frame. The North American plate drifts roughly 2-3 cm per year relative to the global frame, so the two datums have been diverging since they were aligned in 1986.

The cumulative offset has now reached roughly 1-2 m in CONUS. NAD83 is the official datum of US federal mapping and survey work; WGS-84 is the GPS datum. Coordinates collected in WGS-84 must be transformed to NAD83 (or specifically NAD83(2011)) when integrating with US federal datasets — the authoritative tool is NGS NCAT. The forthcoming NATRF2022 modernization (NSRS 2022 successor frame) will align NAD83's replacement more tightly with WGS-84 and ITRF.

EPSG codes built on WGS-84

WGS-84 is a datum; many distinct CRSs share it. Recognising which EPSG code a coordinate is in matters because the coordinate system behind the datum determines units, axis order and dimensionality.

EPSG:4326 is the workhorse, but EPSG:4979 (3D geographic) and EPSG:4978 (geocentric Cartesian) come up in geodetic computation because they are mathematically cleaner than 2D geographic.

Heights in WGS-84: ellipsoidal vs orthometric

WGS-84 reports two kinds of height, and the difference matters for elevation tools, aviation, and any work that touches mean sea level.

EGM2008 publishes the geoid undulation N globally with sub-decimetre accuracy at the spherical-harmonic resolution of degree/order 2190. Geoid undulation ranges from about −106 m near the south of India to +85 m near Iceland and New Guinea — a 190-metre swing globally. A raw GPS height of "100 m" near Iceland actually corresponds to about 15 m above mean sea level; the same raw height in India corresponds to about 206 m above sea level.

Common misconceptions